This invention relates generally to optical filters. More particularly, this invention relates to optical filters employing the atomic resonance transitions found in alkali metal vapor atoms wherein inert gases (namely helium) are used as "buffers" to reduce radiation trapping and pulse stretching. The present invention thus minimizes the response time between the excitation of metallic vapor atoms by uv or visible light and the subsequent availability for detection of infrared radiation emitted by the decay of the excited alkali metal vapor atoms.
The quantum limited optical atomic resonance filter detector or QLORD filter detector described in U.S. Pat. No. 4,292,526 (all of the contents of which are incorporated herein by reference) uses atomic resonance transitions to take incoming light at wavelengths in the visible, absorb it, and reemit the light at infrared wavelengths. These atomic resonance filters employ various metal vapors to absorb very narrow band optical radiation in the visible region of the optical spectrum and subsequently emit infrared radiation. Incoming light is transmitted through a high pass filter, and the infrared emitted light is transmitted through a low pass filter. By use of this out of band processing, very low noise levels can be achieved in addition to the intrinsic narrow linewidth of the acceptance frequency, and the 2.pi. an acceptance angle.
Unfortunately, the quantum limited optical resonance filter detector of U.S. Pat. No. 4,292,526 is associated with excessive pulse stretching (7-8 microseconds in the case of Cesium vapor) caused by radiation trapping within the metal vapor. It will be appreciated that a sharper short emitted pulse is highly preferred over a stretched pulse. This radiation trapping has been analyzed and it has been determined that since a pulse of light travels some 10,000 feet in 10 microseconds, light trapping can seriously limit the performance of the detector for numerous applications.